Lewis, Gilbert Newton (1875-1946)

US theoretical chemist who defined a base as a substance that supplies a pair of electrons for a chemical bond, and an acid as a substance that accepts such a pair. He also set out the electronic theory of valency and in thermodynamics listed the free energies of 143 substances.

Lewis was born in Weymouth, Massachusetts, and studied at Nebraska and Harvard. He worked at the Massachusetts Institute of Technology (MIT) 1905-1912, and spent the rest of his career at the University of California, Berkeley.

In 1916, Lewis began his pioneering work on valency. He postulated that the atoms of elements whose atomic mass is higher than helium's have inner shells of electrons with the structure of the preceding noble gas (rare gas). The valency electrons lie outside these shells and easily form ionic bonds, preferably covalent bonds. He also drew attention to the unusual properties of molecules that have an odd number of electrons, such as nitric oxide (nitrogen monoxide, NO).

In his later years he carried out studies on the excited electron states of organic molecules, contributing to the understanding of the colour of organic substances and the complex phenomena of phosphorescence and fluorescence.

Lewis's main works were both published 1923: Valence and the Structure of Atoms and Molecules and (with Merle Randall) Thermodynamics and the Free Energy of Chemical Substances.